Connection Admission Control is one of a number of known techniques for managing and controlling traffic and congestion in connection-orientated networks. In particular, it is used in ATM (Asynchronous Transfer Mode) networks to provide quality of services (QOS) guarantees. It is not limited to use in ATM networks.
Connection Admission Control (CAC) procedures are used to decide if a request for an ATM connection can be accepted, based on the network capacity and the attributes of both the requested connection and existing connections. It is important that there is always enough bandwidth so that quality of service guarantees for the existing connections and the requested connections, can be met.
CAC procedures may be used at an access node at the edge of an ATM network to enable control of access to the entire route through the ATM network as route selection is made. A second level, may be used at each node along the selected route through the ATM network, to confirm that a respective link beyond that node, can admit the connection. This is also called flow control, but for the present purposes, it is intended to be encompassed by the term admission control.
A useful summary of known admission techniques is contained in an article by Perros and Khaled in IEEE communications magazine November 1996, "Call Admission Control Schemes, a Review". Conventionally, when a CAC algorithm is used, for a link having numerous identified channels, one of these channels will be allocated to the information flow being admitted. As channels can be allocated from either end of the link, there is a risk of a channel being simultaneously allocated by both ends of different information flows. In this case, there is no mechanism to prevent or recover from this, and both information flows would be transmitted but neither would be received. Where the information flows are connections, they would both or either be dropped and the information would be lost.
This has been accepted up to now as the risk is usually very low, depending on the number of free channels, the mechanism for finding and allocating a free channel, and the delay (transmission delay and processing delay) in one side alerting the other side that a channel is no longer free.
However, if it desired to run a network at closer to maximum capacity, and as guarantees of quality of service become more important and more valuable, the inventors have regarded this risk of simultaneous allocation as a potential problem.
Another potential problem not addressed by conventional CAC schemes is where a change of speed, or change in some other characteristic of a connection, occurs after the connection has been admitted. There are many examples of this. In handling connections in the form of calls from the PSTN, a codec may assume a call to be voice and thus codable at a low bit rate, e.g. 8 kb/sec. Once set up and admitted to a network by a CAC algorithm, it turns out to be a fax or a modem call which needs the full 64 kb/sec. Thus, when in band DTMF (dual tone multiple frequency) tones are detected which may show it is not a voice call, the codec rate may change and thus one of the criteria used by the CAC alogorithm may change.
If the CAC algorithm showed the network to be close to the threshold for the maximum allowed bandwidth, it is possible that a change in an existing connection will exceed the threshold, and the change, despite having been admitted at first, will not be admitted by the CAC algorithm.
The connection would be dropped. This risk has been tolerated, but again, where it is desired to run a network close to maximum capacity, and as guarantees of quality of service become more important, the inventors have regarded this risk as a potential problem.